1. Field of the Invention
The present invention relates to a seat frame, and more particularly to a lightweight seat frame construction for vehicles which facilitates operation of the foot pedals.
2. Description of the Prior Art
In a conventional seat, generally a seat cushion mounted on a seat frame is provided for flexibly supporting a passenger. Further, a support structure is located on a vehicle floor and attached to a base portion of the seat frame. In recent years, seat frames commonly are made of plastic to provide a lightweight structure. For example, in U.S. Pat. No. 4,065,182 to Braniff et al. the seat cushion is directly mounted on a plastic seat frame and the rear surface of a backrest cushion is directly supported by a backrest frame made of plastic. In Japanese patent publication No. 55-118715 (1980), a spring mechanism is provided on a plastic seat frame and the lower surface of a seat cushion is supported by the spring mechanism, while or the rear surface of a backrest cushion is supported by another spring mechanism.
Generally, a vehicle seat provides a varying degree of retraction as measured from forward to rearward along the surface of the seat in response to the weight of the seated passenger. This characteristic, which is defined as the spring constant, is shown in FIG. 2. The maximum spring constant, depicted as point "A", is located forward of the point "HP", which supports the buttocks of the seated passenger. The spring constant is gradually decreased forward of point A and is minimum at the front end of the seat. The spring constant is also slightly decreased immediately rearward from point A, before remaining constant. Accordingly, the forward slip of the buttocks of the passenger along the upper surface of the seat is blocked at the point A, while actuation of the foot pedals is somewhat smoothed by decreasing the supporting force of the lap at the front end of the seat.
Nevertheless, the spring characteristics of the seat is defined by the hardness and the thickness of the seat cushion. As such, the seat cushion must be thickened in order to become comfortable and able to absorb the vibration of the seat from the road surface, particularly when the seat on which the seat cushion is directly mounted on the seat frame made of plastic. The seat having the foregoing construction is uneconomical and also prevents smooth actuation of the foot pedals. The latter results because the passenger tends to sink within the thickened elastic seat cushion increasing the tension on the edges of the seat. Thus, smooth actuation of the foot pedals is limited by the thickened portion at the front edge of the seat cushion.